In the case where user's state is estimated in each event (trial) on the basis of an event-related potential and is applied to an appliance, accuracy of user's state estimation based on the event-related potential is significant. As described in Non-patent Document 1, the amplitude of the event-related potential is so small, approximately one tenth of a background electroencephalogram, that it is difficult to identify it by a method of simply subjecting the potential waveform to a threshold processing.
There is listed Patent Document 1 as a conventional technique for applying user's state estimated on the basis of the event-related potential to an interface with an appliance. In Patent Document 1, user's state when user's expectation of appliance's operation is different from actual appliance's operation is defined as disappointment, and user's disappointed state is detected by referring to a disappointment signal component appearing in the event-related potential as an index. Immediate (within approximately 600 ms) detection of the user's disappointment by making use of the feature of the event-related potential achieves interactive service change. In Patent Document 1, disappointment is detected by a discriminant analysis in which a Mahalanobis distance is calculated from a correlation coefficient calculated with the use of an average waveform under a disappointment state obtained by about 20 disappointment samples as a template.
Besides, Patent Document 2 and Patent Document 3 propose techniques for reducing noise mixed with an electroencephalogram signal. In Patent Document 2, a threshold value is determined by frequency analysis, wavelet, template, covariance, or the like. Patent Document 3 refers to a method using a band-pass filter in which reduction in mixed noise is achieved to allow a desired component to be extracted from the electroencephalogram signal.    Patent Document 1: WO2005/001677    Patent Document 2: Japanese Patent Application Laid Open Publication No. 10-80409A    Patent Document 3: Japanese Patent Application Laid Open Publication No. 8-117199A    Non-patent Document 1: Kaga et al., “Manual of Event-related potential (ERP) -Focusing on P300-,” published by Shinohara Shuppan Shinsha, 1995    Non-patent Document 2: “Guideline Proposal of Evoked Potential Measurement,” revised in 1997